A finished video presentation is rarely one continuous scene shot from one camera. More commonly, video presentations are a series of short scenes assembled together through an edit process to form the final presentation. Unlike motion picture film which is edited by physically cutting and splicing film segments together, video material is edited by electronically cutting and splicing together segments of video signals.
A video switcher is an electronic device used by the video industry to execute a form of editing wherein video material from a variety of sources are assembled into a finished video presentation. Video material is provided to the video switcher in the form of input video signals from a variety of video sources, such as video tape recorders, optical disk plays, and video cameras. The video switcher typically selects for presentation as an output video signal one or more of the input video signals from the available video sources. This selection is accomplished by switching from one input video signal to another signal, with the time of occurrence of the switch operator. This output video signal is utilized in a variety of ways such as recording by a video tape recorder, displaying on a video monitor, or broadcasting to viewers.
This switch between input video sources can take many forms and often involves several different switching techniques. The most straight forward of the switching techniques is the simple switch between different video signal sources. In such switchers, the output of the switcher is instantaneously switched between the input video signals and only one of the input video signals is present at any one instant at the output of the switcher. To a viewer of the output signal displayed on a monitor or television receiver, the display changes instantaneously from one input video signal to the other different input video signal.
Another technique of effecting a switch between video sources is called a dissolve. A dissolve is a gradual transition between different video signals, with the different video signals appearing simultaneously in the output provided for display during part of the transition interval. The gradual transition is accomplished through adjustable video signal gain controllers, commonly variable attenuators, and a signal combiner, which additively combine and present to an output the gain adjusted video signals provided by the gain controllers. Usually, a dissolve begins with the output video signal being entirely composed of a first input video signal. At a rate determined by the operator, the gain on the first input video signal is reduced to zero, while the gain on a second input video signal is raised from zero to the level originally set for the first signal. To a viewer of the output signal displayed on a monitor, the picture based on the first input signal appears to dim, while the picture based on the second input video source appears very dim at first and then appears to brighten until only the picture based on the second input video source is visible. At the midpoint of the switch, both signals are at an equal gain of one half and the pictures resulting from the displays of the signals are both visible simultaneously on the monitor, each at one-half its normal brightness.
Yet another technique of effecting a switch between video sources is called a wipe. A wipe is created by generating a control signal which is used by a video mixer to switch between input video signals on a line by line basis, such that a line of output video may be composed of parts of both input video signals. This control signal is usually based on the output of ramp generators which are processed by a ramp combiner, which manipulates the ramps and then combines them so as to form a variety of control signals. During a wipe, both input video signals are present in the output video signal. Unlike a dissolve, where both complete input signals, at reduced gains, are additively combined to form the output video signal, in a wipe both signals are used at full gain, but only part of each signal corresponding to spatial segments of a display are present in the output video signal. Generally, the input video signals do not coexist in the same display spatial segment corresponding to the part of the output video signal. This concept is better understood from the standpoint of viewing the display of the output signal on a monitor. The wipe often begins with a first input video signal, when displayed on a monitor, spatially occupying the entire display area of the monitor. As the wipe progresses, the second input video signal displaces the first input video signal, spatially, in part of the display area of the monitor. The area displaying the second input video signal grows according to a selected pattern as the area displaying the first input video signal reduces according to a corresponding pattern, with the patterns determined according to operator inputs. The wipe is completed when the display area on the monitor is entirely occupied by a display of the second input video signal.
A control signal is used to prevent coupling to the output, input video signals areas that are not to be displayed on the monitor. This control signal is simply turned on or off. When on, the input signal is coupled to the output, and when off, the input signal is prevented from being coupled to the output. Because one of the input video signals is presented at the output video signal of the switcher, the control signal for the first input video signal is the reverse of the control signal for the second input video signal.
An example of a simple wipe is the vertical line wipe. When viewed on a monitor, a transition boundary in the form of a vertical line between display areas on the monitor, defines the separation between the displays of the the first and second input video signals. As the vertical line wipe is executed, this transition boundary moves horizontally across the display area wiping from a picture based on the first input video signal to a picture based on the second video input signal. It is the form that this transition boundary takes, that determines what type of wipe is executed. For example, circle wipes have transition boundaries that form the shape of a box and circle, respectively. The greater the variety of wipes a switcher can execute, greater its value.
An unusual, but highly desirable type of wipe is called the rotating clock wipe. To the viewer, the transition boundary extends from a point generally in the central region of the display area of the monitor outward to one of the edges of the display area of the monitor. The end of the transition boundary located in the central region of the display area of the monitor remains fixed while the end located at the edge of the display area sweeps around the outer edge of the monitor display area, like a hand of a clock. The transition boundary proceeds from a starting position, and the display area changes from a first input video signal to a second video signal as the transition boundary rotates about the display area of the monitor back to the starting position. The start position, direction of rotation of the transition boundary, and the rate of rotation are all entered by the operator.
It is common to discuss a rotating clock wipe in terms of a start angle and a current rotation angle. The line of reference most commonly used, from which these angles are measured, is a vertical line extending generally from the central region of the monitor display area to the outer edge of the monitor display. A clock wipe that had a transition boundary which started with its outside end pointed straight up on the monitor display area would have a start angle of 0.degree.. If the outside end pointed horizontally to the right on the monitor screen, the start angle would be 90.degree., and so on. The current rotation angle is measured from the start angle and indicates the current location of the transition boundary. At the beginning of the wipe, the current rotation angle is 0.degree., and at the end of the wipe, the current rotation angle is 360.degree..
Generating a rotating clock wipe requires complex, specialized circuitry. The current methods use a segmented approach to produce the wipe, that is the wipe is generated in small pieces instead of one continuous wipe. The apparatus used to produce this wipe produces a transition boundary that can only be rotated from a start angle of 0.degree. to 90.degree.. A complete 360.degree. wipe is generated in 90.degree. segments. Additional hardware is utilized to translate the 0.degree. to 90.degree. segment into the three other segments of the circle. A complete wipe is generated by rotating the transition boundary from a start angle of 0.degree. to 90.degree.. The apparatus is then reset back to 0.degree. and the translation hardware translates the transition boundary to 90.degree. and the 0.degree. to 90.degree. is re-executed from 90.degree. to 180.degree.. The result of this segmented approach is jerky motion at various points throughout the wipe.
As the apparatus only allows the transition boundary to be started at the beginning of a segment, clock wipes may only be started at the beginning of each segment, thus limiting the operators freedom as to where to start the wipe.
As a result of these problems, there is a need for an apparatus or method that can produce a non-segmented continuous rotating clock wipe. Such an apparatus or method desirably also should perform a rotating clock wipe starting at any position.